From: gdemont AT my-deja DOT com
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Performance 2.7.2 -> 2.8.1
Date: Thu, 19 Oct 2000 20:51:12 GMT
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Hans-Bernhard Broeker :

> *How much* slower? It's really hard to speak about optimization
> differences if their size is not known. Is it: a percent slower?
> Fifty?  Taking 5 times as long?

No no, be reassured: a few percents.

> And what's that code doing, in the first place?

Basically, a 3D engine and a unzipper without ASM insertions.
I could annoy you with frames-per-second log files but well...
I'm not writing a very serious article in Journal of Computational
Physics about it. I was just curious.

> '-funroll-loops' hardly does you any good, on any x86 type machine. It
> increases the code size, and as soon as the amount of code being
> looped over (the 'active set') goes beyond the size of the 1st level
> Cache, you'll receive a noticeable performance degradation. Same as
> you cross other size barriers. Pentium-class machines are good enough
> at branching (and branch prediction, in particular) that unrolling
> loops doesn't gain you terribly much, anyway, before the cache
> coherency loss strikes back.

I'm well aware about the bad cache windowing when code size increase,
especially when you let the auto-inlining option on, but trust me,
in *that* case '-funroll-loops' has a positive effect (up to 10%).

Anyway, thank you.

G.


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